(57) In a power assisted steering system for a vehicle, an operator controlled steering
shaft (24) is connected directly to at least one steered wheel of the vehicle. An
electric motor (12) is connected to the steering shaft (24) via a ratio reduction
means (14). A clutch (28) is interposed between the motor (12) and the ratio reduction
eans (14) to connect and disconnect the motor from the steering shaft (24). A control
circuit (36) senses torque and direction of rotation of the steering shaft (24) by
the operator and operates the motor (12) to provide power steering assistance. The
control circuit (36) also monitors the operation of the system and, when the system
is operating properly, keeps the clutch (28) in engagement to transmit torque from
the motor (12) to the steering shaft (24). When the system is diagnosed to be operating
improperly, the clutch (28) is rapidly disengaged and the motor (12) is effectively
isolated from the steering shaft (24).
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